This proposal is concerned with the general topic of biological electron transfer reactions. Specifically it involves developing well-defined models for the structurally characterized reaction centers of R. vizidis and R. sphaeroides and using these to probe various mechanistic issues currently being debated within the photosynthetic and biological electron transfer community. A variety of selectively metalated quinone substituted dimers, wherein the central metal, relative subunit orientations, and over- all driving force may be varied in a systematic manner, will be prepared, and several related but more elaborate quinone substituted porphyrin trimers, tetramers, and hexamers synthesized. Static fluorescence quenching, single photon counting, and transient absorption studies of these new systems will provide insight into the currently unanswered questions of how changes in local molecular geometries, external environment, intermediate state energetics, and driving force influence the rates of multistep photoinduced electron transfer reactions. Particular emphasis will be placed on ascertaining whether electron transfer reactions take place through space or through bonds, and determining the extent to which "superexchange" mechanisms play a role in mediating multistep electron transfer processes. This work is thus expected to provide insight not only into particular questions associated with bacterial photosynthesis but also general issues relevant to all multistep biological electron transfer processes. Electron transfer reactions play a central role in biology. They are a crucial component in a wide range of enzymatic processes and play a critical role in both photosynthesis and oxidative phosphorylation. The latter process is, of course, required for all higher life forms, including humans. As a result, interfering with the electron transport sequence of oxidative phosphorylation can have serious consequences: cyanide anion, for instance, which binds directly to the ferric form of cytochrome c oxidase, is highly toxic. Moreover, factors which perturb heme synthesis, and hence reduce the efficiency of electron transport, also give rise to symptoms of long-term toxicity. Included in this category are certain hereditary diseases, such as acute intermittent porphyria, and chronic exposure to various heavy metals such as lead and arsenic. The proposed work will provide detailed mechanistic information about biological electron transport processes. As such, it may lead ultimately to the development of new treatments for these induced or congenital disorders.